The technique for converting signals to modulate amplitude is well known. Signals may be conditioned or enhanced. Signal conditioning is the process of modifying or adapting a signal to match a specific purpose. For example, a typical system in which signals are conditioned includes the detection and filtering of an alternating-current wave for indirect voltage measurement using a direct current meter. Another example is the implementation of an equalizer in a high-fidelity system. Signal enhancement relates to a process in which the quality of decipherabilty of a signal is improved. For example in radio-frequency (RF) receivers, signal enhancement is implemented to optimize the signal-to-interference ratio. Further, in high-fidelity audio receivers, signal enhancement is used to reduce noise and adapt the sound to a listener's preferences.
Signal processing also utilizes modulation techniques to condition signals. Generally, modulation techniques implement a synthesizer to generate a desired signal. Modulation in general refers to the process of changing the characteristics of an electromagnetic wave in a manner to convey communication or other intelligence. Several difficult aspects of modulation are encountered in the art when more data are required at a rate higher than that allowed by normal modulation.
Traditionally, in order to transfer signals at a high rate of speed, modulation based signal processing techniques employ various complex systems such as up conversions, several filters, synthesizers and oscillators. These systems, to a large extent, encounter frequency pulling and are prone to unreliable and distorted signal output. Specifically, spurious responses from mixing devices contribute to output signal corruptions. Further, prior art systems require significant modifications and upgrade for integration with other multichannel data command units specially if frequency pulling is anticipated. Many synthesized radio units, which implement amplitude modulation, require a better and efficient topology to overcome the aforementioned problems and also to transmit the information contained in the signals at high fidelity and quality.
FIG. 1 is a prior art schematic representing a circuit implemented to reduce frequency pulling. The circuit includes a reference signal source 10 which is connected to a plurality of frequency dividers 11. The output from each frequency divider 11 is fed into phase detectors 12. Each phase detector 12 is connected to low units, which implement amplitude modulation, require a better and efficient pass filter (LPF) 13 on the output side. LPFs 13 are then connected to VCO115a and VCO215b, respectively. The VCOs, 15a, 15b produce frequencies which are fed into mixer 16. A portion of VCO115a and VCO215b output is fed back into phase detectors 12 via frequency dividers 14. The output from mixer 16 is directed to pre-transmission filter 17. Thereafter, the signal is directed through amplifier 18 to antenna 19. The frequency output at antenna 19 is a sum and or difference of the frequency outputs from VCO115a and VCO215b.
The prior art utilizes up and down conversion of the signal source to reach an intermediate frequency (IF). The IF is the output frequency from the mixer stage. High gain and only moderate selectivity can be obtained because the high Q factor required on the filters due to the high upconverted frequency.
One of the many limitations of the prior art is that the architecture for reducing carrier frequency pulling is very complex. Specifically, the scheme requires several filters, oscillators and mixers to eliminate spurious and non-related harmonics from the transmitter. However, these components generate local interferences, both individually and in combinations, which further distort the information carried in the signal and the output frequency.
Accordingly there is a need to provide a reliable and efficient circuit to attenuate oscillator frequency pulling including elimination of spurious and providing related harmonic isolation from the output of an amplitude modulated VCO. It turns out that output frequencies which are harmonically related to either VCO1 or VCO2 (15A or 15B) tend to also pull the VCO's frequency as well.